The present invention relates to plasma arc torches (xe2x80x9cplasma torchesxe2x80x9d) as used in furnace melting, for example.
One type of plasma torch employs a cylindrical electrode with a center bore; a gas-constricting nozzle at and spaced from a front end of the electrode; a so-called xe2x80x9cswirl chamberxe2x80x9d which surrounds the space between the electrode and the nozzle; and an arrangement for generating a vortical flow of pressurized gas which flows from the swirl chamber back into the electrode bore and swirls forwardly through the discharge opening of the nozzle.
A plasma torch develops heat with an arc which is drawn between the electrode and the workpiece (called the transferred mode). Alternatively, heat may be developed between a torch electrode and a second electrode (called non-transferred mode). The transferred mode is usually more efficient for heating conductive solids and/or liquids because energy transfers directly from the torch to the workpiece, rather than partially dissipating to a separate electrode, and the present invention is especially concerned with transferred mode torches.
When an arc is struck between the electrode and a workpiece outside of the nozzle, the gas carrying the arc current becomes ionized, thereby forming a plasma which is expelled through a constricting discharge opening of the nozzle as a swirling, superheated plasma jet that melts the workpiece. The swirling gas also helps to protect the electrode from erosion or contamination because the point on the electrode from which the arc emanates (arc termination point) tends to spin with the arc gas instead of remaining stationary.
U.S. Pat. No. 5,239,162, the disclosure of which is incorporated herein by reference, discloses an improved plasma arc torch and illustrates, in FIG. 3 thereof (which is reproduced herein), a swirl chamber principally formed between the front end of the electrode and the torch nozzle. FIG. 3 of the patent illustrates, as is conventional, locating a plurality of primary (plasma) gas discharge ports so that they direct a plurality of individual gas flows generally tangentially into the swirl chamber to rotate or spin the gas as it flows from the swirl chamber through the torch and the discharge opening of the nozzle towards the workpiece that is to be melted. Typically, such torches include an annular space (not shown in U.S. Pat. No. 5,239,162) which is concentric with the electrode and extends from the front end thereof in an aft direction. The aft end of the annulus is closed, located some distance from the front end of the electrode, and includes secondary gas discharge ports which may be located adjacent the aft end or at intermediate positions along the annulus and direct individual gas flows into the annulus and towards the swirl chamber. Gas from the secondary discharge ports combines with the gas from the primary ports and smoothes the vortical flow, which in turn improves the operation of the torch.
While in the swirl chamber, the gas flows from the discharge ports spin at a relatively high rate and create a vortex flow into the nozzle discharge opening as well as a pressure gradient in which the pressure is largest at the outer boundary of the rotating gas flow and decreases towards the rotational center. A corresponding radial pressure gradient is also present inside the annulus just above the swirl chamber so that the pressure in the annulus along its outer diameter is slightly larger than the pressure along the inner diameter of the annulus.
Such torches worked well for their traditional applications, the melting of metals. Recently, plasma torch furnaces using transferred mode plasma torches have been used for heating and melting materials in several other industries, including the treatment (melting and/or incineration) of waste. Transferred mode plasma torches use relatively lesser gas flow rates (as compared to non-transferred mode plasma torches), and during heating and melting of waste materials severe abrasion was encountered from particles which were drawn by the low pressure near the axis of the swirling gas into the swirl chamber, where centrifugal forces gyrate them radially outward towards the outer wall of the swirl chamber. Particles in past torches were also drawn along the outside wall into the annulus above the swirl chamber, apparently because a fraction of the gas from the secondary discharge ports recirculated back into the annulus. Centrifugal forces of the gas spinning in the annulus forced the particles radially outward against the outer wall of the annulus and caused extensive abrasion along the outer annulus wall. The problem is so severe that the affected components of the torch, especially what is commonly referred to as the gas ring, may become unusable and require replacement after as little as 10-30 service hours.
Replacing the gas ring is time-consuming because the furnace and the plasma torch must first be cooled down. Only thereafter can the worn gas ring (and any other worn parts of the torch) be replaced. The frequent replacement of the gas ring is costly. In addition, each time the gas ring must be replaced the furnace experiences a prolonged downtime, which significantly reduces the efficiency of the furnace and further increases costs.
The present invention overcomes the above-described problems encountered with prior art plasma torches by eliminating all (plasma) gas discharge ports in the swirl chamber of the torch, that is, forward of the annulus above the swirl chamber. Instead, all gas injection ports are located (preferably in a single plane that is perpendicular to the annulus) just forward or short of the closed aft end of the annulus surrounding the electrode. This generates a relatively large, positive pressure gradient from the aft end to the forward end of the annulus and induces a positive forward gas flow over substantially the entire cross-section of the annulus, thereby preventing the heretofore encountered backflow of gas which could carry abrading particles into the annulus. Thus, the present invention prevents the particles from entering the annulus in the first place. In addition, any particles that may have entered the annulus are positively moved out of it towards the swirl chamber. As a result, gas ring erosion from spinning particles is reduced to such an extent that instead of the heretofore typical 10-30 hour service life, gas ring service life is increased by a factor of 10 or more. This significantly reduces replacement costs and the frequency of costly furnace downtimes.
Generally speaking, therefore, the present invention provides a swirl flow plasma arc torch which has an elongated electrode with an open front end and a nozzle with a plasma discharge opening that is coaxial with the electrode. In the preferred embodiment, a mounting arrangement for the electrode includes a ceramic ring at the front end of the electrode and a gas ring which concentrically surrounds the ceramic ring. A forward portion of the gas ring, the forward end of the electrode, and the nozzle define the swirl chamber of the torch, and opposing, spaced-apart concentric cylindrical surfaces of the ceramic ring and the gas ring, respectively, form the annulus which extends rearwardly from the swirl chamber. The ceramic ring closes the aft end of the annulus, and the gas ring houses a plurality, typically 4 to 8, of plasma gas injection ports which preferably lie in a single plane located immediately forward of the aft end of the annulus.
In use, gas is directed from the injection ports generally tangentially near the rear of the annulus to impart rotation to the gas after it leaves the ports and as it flows toward the swirl chamber. Best results are obtained when the plasma torch establishes a uniform, swirling plasma gas flow in the swirl chamber and the nozzle discharge opening. This is attained by making the annulus sufficiently long so that the injected gas performs between 5-20 revolutions as it spirals forwardly before it enters the swirl chamber as a uniform, single mass gas flow.
Due to the absence of any appreciable number of particles in the annulus, and their prompt expulsion therefrom should they enter, abrasion of the gas ring is greatly reduced, its service life is correspondingly lengthened, and the frequency of costly furnace downtimes to change gas rings is significantly lowered. Accordingly, the present invention substantially improves the economics of operating furnaces heated by plasma torch.